1. Field of the Invention
The present invention relates to a thermoelectric material.
2. Related Art
In recent years, thermoelectric generation as a new energy conversion technology having low environmental impact has been recognized. A Seebeck effect, which converts heat energy into electrical energy by a temperature difference between materials using a p-type thermoelectric material and a n-type thermoelectric material, is used in such technology.
A thermoelectric conversion efficiency performance index Z is represented by the following Equation 1 using Seebeck coefficient S, electric resistance ρ, and thermal conductivity κ as characteristics of the materials.Z=S2/ρκ  Equation 1:
Wherein S2/ρ is referred to as an output factor, which is an indicator of a thermoelectric characteristic showing the extent of an electric current. Accordingly, material having a high Seebeck coefficient and a low electrical resistivity and thermal conductivity is preferable so as to enhance the thermoelectric conversion efficiency.
Semiconductor sintered materials such as Bi—Te system, Pb—Te system and Si—Ge system account for much of the conventional thermoelectric materials. Recently, Co—Sb skutterudite system and layered oxide or the like have been studied. A thermoelectric cooling technology that uses Bi—Te system and applies a Peltier effect has already been put to practical use in an electric refrigerator and a temperature-control device. A generating technology using the Seebeck effect has been put to practical use only in specialized applications such as a satellite power supply, due to the issues of cost posed by growing in size or the like.
B (boron) and Se (selenium) must be added to the conventional thermoelectric semiconductor because p-type and n-type thermoelectric materials are used in a Bi—Te system. Se, Te (tellurium) and Pb (lead) are detrimental elements and they are unfavorable in terms of the global environment. Materials are expensive as the above elements and Ge (germanium) or the like are scarce.
In the Fe—V—Al system of the present invention, it is well known that the code of the Seebeck coefficient significantly changes by slightly scooting down the Fermi level due to a slight change in temperature of V (vanadium) in Fe2VAl (Journal of Alloys and Compounds, 329 (2001), p63-68). Because the Fe—V—Al system is an iron-containing material unlike the conventional semiconducting material, the thermoelectric material can be produced by adjusting ratios of atomic concentration to objective compositions and by casting it.
In the Fe—V—Al system of the present invention, when a part of Fe in Fe3Al having DO3-type crystal construction is replaced with V (vanadium), Fe3Al is changed to a more regular Heusler-type L21 crystal construction (Fe2VAl), thereby forming a sharp pseudogap to the Fermi level. In the Fe—V—Al system suggested hitherto, the thermoelectric characteristic is improved by replacing a part of Fe in Fe2VAl with Mn or Cr (see P2003-197985), by replacing a part of V in Fe2VAl with Ti or Mo (see JP2004-253618) and by replacing a part of Al in Fe2VAl with Si, Ge or Sn (see JP2004-253618).